Effect of Negative permittivity?

[indio007]

Does anyone know how quantum theory accounts for a ε value that is negative?
Wouldn't this change probability densities?

[olgranpappy]

Does anyone know how quantum theory accounts for a ε value that is negative?
Wouldn't this change probability densities?

You can have a negative permittivity, no problem. For example, an implicit equation for the dispersion of surface plasmons is given by setting the (frequency and wavelength dependent) permittivity to -1.

[DrDu]

Usually a negative permittivity arises just above a resonance where the polarization is 180 degree out of phase with the driving field. This can already be understood in the case of a classical driven harmonic oscillator.